1. Field of the Invention
The present invention relates to an integrated control apparatus for a vehicle capable of enhancing vehicular running stability.
2. Description of the Related Art
A conventionally known integrated control apparatus for a vehicle is disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-109866. The integrated control apparatus for a vehicle combines braking force control with 4WS (4-wheel steering) steering-angle control and performs target follow-up control by use of yaw rate feedback, thereby enhancing vehicle stability during, for example, cornering. Japanese Patent Application Laid-Open (kokai) No. 2002-254964 also discloses another known integrated control apparatus for a vehicle. According to the technique of this publication, feedforward control is applied to actuators, which are objects of control, in a steering system, a brake system, and a drive system on the basis of target longitudinal forces (drive force and braking force), a target side force, and a target yaw moment of a vehicle, thereby enhancing vehicle stability. However, the integrated control apparatus disclosed in the publications do not reflect the state of a road surface on control. As a result, in some cases, expected vehicle stability is not attained.
Meanwhile, Japanese Laid-Open Patent Application No. 10-310042 discloses a vehicle control apparatus that reflects the behavioral condition of a vehicle and the state of the road surface on control. According to this technique, the maximum resultant of forces that can be generated between a wheel and the road surface is set as the radius of a friction circle; and the magnitude and direction of a force that the wheel generates are controlled such that the resultant of forces actually generated between the wheel and the road surface does not exceed the force represented by the set radius of the friction circle. Notably, the resultant of forces that are generated between a wheel (tire) and the road surface in every direction in a horizontal plane cannot exceed the product of the vertical load W at that time and the friction coefficient μ. Thus, the vector of the resultant remains within a circle having the radius μW. This circle is called a “friction circle.”
Therefore, in a tire rotating at a certain slip angle, a resultant Ft of a longitudinal force Fx and a side force Fy, which are generated at the tire's contact surface, is represented as “Ft=√{square root over (Fx2+Fy2)}.”
According to the above-mentioned Japanese Patent Application Laid-Open (kokai) No. 10-310042, when a force on the above-mentioned friction circle is taken as a grip limit force Fg (=μW), the tire margin “Vmrg=1−(Ft/Fg)” is calculated. In accordance with the calculated tire margin, actuators in various systems, such as the steering system and the brake system, are controlled.
In the above-mentioned Japanese Patent Application Laid-Open (kokai) No. 10-310042, in order to obtain the grip limit force Fg, the friction coefficient of the road surface must be estimated. However, accurately estimating the current friction coefficient μ of the road surface is difficult. Also, estimating a momentary change in the friction coefficient of the road surface is difficult. Therefore, accurately estimating the grip limit force Fg, which varies according to the state of the road surface, is difficult. For this reason, when the tire margin, which is calculated by use of an estimated value of the grip limit force Fg, is used for control in the critical region of the vehicle (where the tire grips the road surface with a force near the grip limit force), no problem arises. However, using the tire margin for control before the critical region of the vehicle is reached is inadequate.